Meepo spells are considered too weak, since they are designed to work with a hero who can cast them 2-5x in a row.

Infest is disabled because you don't get Control, and the devs consider the spell too weak without the sub-spell.

These are the official reasons given.

Walrus Kick: Would be innate to Tusk, if enabled. Strangely, Eyes in the Forest and Burrow are enabled for Treant/Nyx. Eyes loses half its power without Overgrowth, and Burrow is pretty much useless without Impale/Mana Burn/Spiked Carapace. Walrus Kick is the only spell which is completely independent from other spells, but is the one disabled.

Beastmaster is the only hero disabled for no reason at all. He has 4 spells without any sub-spells, and all of them are independent from each other.